Study On Improving Gas Barrier Properties Of Paper-based Fiber Materials By Using Chitosan-based Composite Coatings

Paper-based materials are composed of natural plant fibers,which have good reproducibility and biodegradability.However,wood fibers contain a large number of hydrophilic hydroxyl groups and the microfine hierarchical structure of the cell lumen and cell wall of the wood fiber cells themselves can induce capillary action together with the three-dimensional network structure formed by the interweaving of numerous fibers.Therefore,the paper-based material without any treatment cannot produce an effective barrier to water vapor and oxygen,and these gases（water vapor and oxygen）can accelerate the physical,chemical and biological reactions of food products,which in turn shorten their shelf life.In recent years,biodegradable polymer-based composite packaging materials have attracted much attention based on the pressure for ecological protection and resource conservation.Polysaccharide polymer-coated paper-based packaging materials make full use of the good film-forming properties and biocompatibility of polysaccharide polymers and the excellent strength properties of wood fiber materials,and have some potential applications in the field of high water vapor and oxygen barrier materials.In this paper,the effect of different chitosan coating amounts on the gas barrier properties of paper-based materials was firstly investigated by using chitosan as raw material among polysaccharide polymers to determine the optimal coating amount.To further improve the water vapor and oxygen barrier properties of paper matrix composites,the effects of different compounding methods and different ratios of chitosan with other materials（polyvinyl alcohol and beeswax）on the water vapor and oxygen barrier properties of paper matrix composites were investigated.The oxygen barrier property,water vapor barrier property and thermal stability of the coating were examined by oxygen transmission rate tester,water vapor transmission rate tester and thermogravimetric analyzer,respectively;the interaction between chitosan,polyvinyl alcohol and beeswax and the surface and cross-sectional structure of the coated paper were analyzed by FTIR and SEM,respectively;meanwhile,the effects of chitosan and its composite coating on the mechanical properties,Cobb value and water contact angle.The main findings of the study are as follows.（1）The effects of different chitosan coating amounts on the gas barrier performance of paper-based materials were studied to determine the optimal chitosan coating amount.Using commercial A4 printing paper as the base material,the paper matrix composites with different chitosan coating amounts were prepared by selecting different types of squeegees and controlling the concentration of chitosan coating solution to coat chitosan on the paper matrix material surface.The results showed that the oil-proof grade of the paper matrix gradually increased and the air permeability gradually decreased with the increase of chitosan coating amount below 2 g·m^-2.When the chitosan coating amount is 2 g·m^-2,the air permeability of the paper-based material exceeds the detection range of the instrument,and the oil-proof grade reaches the highest grade of 12.The oxygen barrier performance test of the paper-based material shows that when the chitosan coating amount is 4.5 g·m^-2,the oxygen transmission amount of the paper-based material decreases to 20.66 cm³·m^-2·day^-1 to meet the food packaging requirements;the water vapor transmission amount increases with The water vapor transmission rate decreases gradually with the increase of chitosan coating,but the decrease ratio decreases gradually.（2）Chitosan（technical grade）/polyvinyl alcohol（technical grade）was coated on the surface of paper-based materials in different ways,and the effects of different amounts of chitosan and polyvinyl alcohol and different coating methods（blending and layering）on the water vapor barrier,oxygen barrier,Cobb value and mechanical properties of paper-based materials were studied more systematically to evaluate the role and synergistic effects of the two components in improving the gas barrier properties of paper-based materials.The results show that as the ratio of chitosan/polyvinyl alcohol decreases,the water vapor barrier properties of the paper-based composites gradually increase,while the oxygen barrier properties gradually decrease.Compared to the CS/PVA blending-coated paper samples,the CS/PVA layering-coated papers have lower water vapor transmission ratio（WVTR）and oxygen transmission ratio（OTR）.When the ratio of CS/PVA is 1:1,the water vapor transmission and oxygen transmission rate of the paper-based composite obtained by the layered coating method were 9.36g·m^-2·day^-1and 31.93 cm³·m^-2·day^-1,which reduced the water vapor transmission by about 92%compared with the pure chitosan-coated paper and was about 68%lower in water vapor transmission and 67%lower in oxygen transmission compared with the paper matrix obtained by mixed coating.（3）Hydrophobic beeswax and chitosan were coated on the surface of paper-based materials in different compounding methods（layered compound and mixed compound）(total coating volume 4.5 g·m^-2)to study the effects of different compounding methods of chitosan/beeswax and different additions of beeswax on the gas barrier properties of paper-based fiber materials.The results showed that the water vapor transmission of coated paper decreased with the increase of beeswax addition.When the addition of beeswax was 15%,the water vapor transmission of layered coated paper-based materials and mixed coated paper-based materials were reduced to 21.87 g·m^-2·day^-1 and 24.13 g·m^-2·day^-1 at50%RH,respectively,which were 80.3%and 78.2%lower than those of pure chitosan coated paper,and the difference between the water vapor transmission of layered coated paper and mixed coated paper was not significant;under the condition of 75%RH,the water vapor transmission amount of layered coated paper samples(163.69 g·m^-2·day^-1)and mixed coated paper samples(242.24 g·m^-2·day^-1)was reduced by 79.4%and 69.5%,respectively,compared with pure chitosan coated paper,while the water vapor transmission amount of layered coated paper was reduced by 32%compared with mixed coated paper.The oxygen transmission of the mixed coated paper-based materials and layered coated paper-based materials increased with the increase of beeswax content and remained as low as 75.15 cm³·m^-2·day^-1 and 101.75 cm³·m^-2·day^-1 for layer-coated and mixing-coated papers at 15%beeswax content.